Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201838

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
238 PM EDT Wed Sep 20 2017

A weak trough will move east of the area tonight and settle along
the southeast coast on Thursday. A gradual increase in more moist,
easterly flow is expected through the latter part of the work week.
Drier and stronger high pressure will settle across the region over
the weekend and likely persist through early next week, as Tropical
Systems Jose and Maria remain off the east coast.


As of 230 PM: A weak ridge of high pressure is still present
over the region, though a well-defined shortwave trough is
centered over SW VA. Vort lobes embedded within the trough are
expected to rotate through the area this aftn and evening. Warm
temps and good lapse rates within the trough have allowed
1500-2000 J/kg of SBCAPE to develop, per SPC mesoanalysis.
Initiation has been spotty so far, with deeper cumulus over the
mountain ridges but also in an area of enhanced convergence
southeast of I-85, just ahead of the trough. Meso models have
not done an especially good job so far today, being overdone
with convection for the most part, though the HRRR has done a
reasonable job of highlighting the two areas where radar echoes
have developed so far. Thus, the revised PoPs are close to the
previous values, blending in a bit of meso model output to
reflect the expected coverage. It is not out of the question
that the most vigorous couple storms could produce a damaging
microburst given the dry air wrapped into the shortwave. Shear
is just enough to expect storms to propagate slowly along
outflows. All this activity will diminish diurnally, despite the
shortwave taking its time moving southeast thru the region
tonight. Under partly cloudy to mostly clear skies, but
continued moist sfc conditions, min temps will remain 4-6
degrees above normal. Expect some fog in the mtn valleys and in
the more fog-prone areas of the Piedmont.

A sharp ridge of high pressure will build over the inland parts
of the eastern CONUS on Thursday, while the shortwave will remain
on our southeast flank. Though the high will bring N to NE flow,
thermal advection is basically neutral, so max temps will remain
6 to 8 degrees above normal. Lapse rates won`t be quite as good in
the wake of the shortwave, but respectable instability still will
develop. Models mainly key on terrain as an initiation mechanism,
leading to chance range PoPs over the mountains and slight chance
in the Piedmont. Profiles feature a bit more moisture aloft but
sfc-midlevel theta-E lapses are progged to be sufficient for a tiny,
but nonzero, threat of damaging winds.


As of 155 PM EDT Wednesday: An upper level trough axis will settle
along the southeast coastline Friday and linger through Saturday, as
a 590 dm ridge axis sets up from the Great Lakes to the MS Valley.
Low level easterly flow continues Friday into Saturday which should
help to focus the highest chance PoPs along the eastern slopes of
the mountains. Thus anticipate mainly diurnal, scattered ridge
top/eastern mountain convection both days with weak forcing and
limited moisture. Drier air moves into the NC Piedmont on this flow
Saturday limiting precip chances there, with better chances
remaining over the mountains, western Upstate and NE GA. SBCAPE
values should steadily recover to 1000 to 1500 J/kg across the
mountains to support a scattered thunder mention. Instability will
be lower elsewhere but high enough for thunder. Highs will be nearly
steady around 5 degrees above normal both days. With lows around 5
degrees above normal Friday morning falling a couple of degrees
Saturday morning.


As of 155 PM EDT Wednesday: the extended fcst picks up at 00z on
Sunday with persistent and highly amplified upper ridging in place
over the Eastern CONUS and steep upper trofing over the west. The
trof will gradually slide eastward thru the period and will begin to
flatten the ridge by the end of the period. At the sfc, very broad
high pressure will be in place to start the period. The remnants of
TS Jose will still be lingering off the New England Coast while TS
Maria is currently forecast to be due east of the Bahamas by early
Sunday. Maria is still expected to remain far enough off the east
coast to not have significant impacts on our fcst area. We will
remain under broad high pressure thru day 7 with predominately dry,
N to NELY low-lvl flow expected to persist. A cold front is expected
to approach the fcst area just beyond the fcst period early Thurs.
As for the sensible fcst, no major changes were needed. The period
should be mostly dry with temperatures starting out about a category
above normal and warming a few more degrees by the end of the period.


At KCLT and Elsewhere: Expecting some SHRA/TSRA this aftn in mainly
two areas, near the Blue Ridge as well as the lower Piedmont. A
TEMPO is included at KCLT and KHKY, and VCSH at KAVL. Otherwise
just low VFR cumulus will be seen; these and the precip will
diminish by sunset. Winds will be quite VRB this aftn, but as a
shortwave trough exits the area and is replaced by high pressure
from the north, they will tend to veer with time, becoming N by
late evening. Fog again is progged in the mtn valleys and outlying
areas of the Piedmont; TEMPO IFR is included at KAVL.

Outlook: Chances for diurnally favored showers and thunderstorms
remain in the fcst Thursday and Friday. Restrictions from early
morning fog across the northern NC Piedmont and foothills, as well
as in the mtn valleys, will also be a concern. Drier weather is
expected to return for the weekend.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  97%     Low   51%     High  81%
KHKY       High 100%     High 100%     High  97%     High  97%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  97%     High  91%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:




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